The present invention relates generally to medical devices and more particularly to self-expanding stents.
Stents have become a common alternative for treating vascular conditions because stenting procedures are considerably less invasive than other alternatives. As an example, stenoses in the coronary arteries have traditionally been treated with bypass surgery. In general, bypass surgery involves splitting the chest bone to open the chest cavity and grafting a replacement vessel onto the heart to bypass the stenosed artery. However, coronary bypass surgery is a very invasive procedure that presents increased risk and requires a long recovery time for the patient. By contrast, stenting procedures are performed transluminally and do not require open surgery. Thus, recovery time is reduced and the risks of surgery are minimized.
Many different types of stents and stenting procedures are possible. In general, however, stents are typically designed as tubular support structures that may be inserted percutaneously and transluminally through a body passageway. Typically, stents are adapted to be compressed and expanded between a smaller and larger diameter. However, other types of stents are designed to have a fixed diameter and are not generally compressible. Although stents may be made from many types of materials, including non-metallic materials and natural tissues, common examples of metallic materials that may be used to make stents include stainless steel and nitinol. Other materials may also be used, such as cobalt-chrome alloys, amorphous metals, tantalum, platinum, gold, titanium, polymers and/or compatible tissues. Typically, stents are implanted within an artery or other passageway by positioning the stent within the lumen to be treated and then expanding the stent from a compressed diameter to an expanded diameter. The ability of the stent to expand from a compressed diameter makes it possible to thread the stent through narrow, tortuous passageways to the area to be treated while the stent is in a relatively small, compressed diameter. Once the stent has been positioned and expanded at the area to be treated, the tubular support structure of the stent contacts and radially supports the inner wall of the passageway. The implanted stent may be used to mechanically prevent the passageway from closing in order to keep the passageway open to facilitate fluid flow through the passageway.
One common type of stent used in medical procedures is the self-expanding stent. Self-expanding stents are usually made of shape memory materials or other elastic materials that act like a spring. Self-expanding stents are increasingly being used by physicians because of their adaptability to a variety of different conditions and procedures. Typical metals used in this type of stent include nitinol and stainless steel. However, other materials may also be used. To facilitate stent implantation, self-expanding stents are normally installed on the end of a delivery catheter in a low profile, compressed state. The stent is typically inserted into a sheath at the end of the catheter, which restrains the stent in the compressed state. The stent and catheter assembly is then guided along a guide wire to the portion of the vessel to be treated using the Seldinger technique, which is well known in the art. Once the catheter and stent are positioned adjacent the portion of the vessel to be treated, the stent is released by pulling, or withdrawing, the sheath rearward. Normally, a stop or other feature is provided on the catheter to prevent the stent from moving rearward with the sheath. After the stent is released from the retaining sheath, the stent springs radially outward to an expanded diameter until the stent contacts and presses against the vessel wall. Generally, self-expanding stents are selected such that the expanded outer diameter of the stent is greater than the inner diameter of the blood vessel. In this way, the continuous outward force of the stent against the inner surface of the blood vessel helps to hold the stent in the deployment location and prevent migration of the stent through the vessel.
Traditionally, self-expanding stents have been used in a number of peripheral arteries in the vascular system due to the elastic characteristic of these stents. However, they may be used in the coronary, carotid, femoral, and renal arteries as well. One advantage of self-expanding stents for peripheral arteries is that stresses from external sources do not permanently deform the stent. As a result, the stent may temporarily deform during unusually harsh stresses and spring back to its expanded state once the stress is relieved. However, self-expanding stents may be used in many other currently known or later developed applications as well.
Self-expanding stents are commonly used in angioplasty, or the mechanical widening of narrowed or completely obstructed blood vessels. Typically, the blood vessels are narrowed or obstructed as a result of arteriolosclerosis or atherosclerosis. Angioplasty is generally performed using a balloon that is tightly folded around a catheter. The catheter is delivered to the treatment site using the aforementioned Seldinger technique, and the balloon is inflated with a fluid, typically saline, contrast, or a mixture thereof. The fluid is injected into the balloon using pressures that are much higher than normal blood pressures until the balloon is inflated to a fixed, predetermined size. This high pressure inflation of the balloon forces the vessel wall at the treatment site to expand in a radially outward direction, thereby widening the obstructed portion of the blood vessel.
Once the blood vessel has been expanded, a self-expanding stent is delivered to the treatment site and deployed into the blood vessel in the above described manner. Because the self-expanding stent acts like a spring, once the stent is released from the delivery catheter it immediately expands to the inner diameter of the blood vessel and continuously exerts outward pressure against the vessel wall. While this outward pressure helps to maintain the position of the stent, the sudden application of outward pressure by the stent against the wall of a vessel that has just undergone balloon expansion may further traumatize the vessel tissue. Such trauma may give rise to potential problems such as hyperplasia, or the abnormal proliferation of cells at the treatment site.
Current research has shown that hyperplastic response to angioplasty or stenting appears to be greatly increased in vessels where the internal elastic lamina (“IEL”) layer of the vessel is ruptured during the angioplasty or stenting procedure. In areas where the IEL is ruptured or damaged, the blood vessel usually exhibits a healing inflammatory response in the form of neointimal growth (abnormal increased growth of cells), which may lead to restenosis, or re-narrowing of the blood vessel. In contrast, areas of the blood vessel where the IEL is left intact tend not to exhibit such neointimal growth. Consequently, it is preferable to leave the IEL intact during the stenting procedure in order to reduce hyperplasia/neointimal growth.
One attempt to reduce stent caused trauma to blood vessel tissue is illustrated by U.S. Pat. No. 6,613,077 to Gilligan et al. Gilligan et al. proposes the use of biodegradable sutures circumferentially wound tightly around the exterior of a stent. After the stent is deployed in a vessel, the constraining sutures restrain the stent from fully expanding. As the sutures begin to biodegrade they yield and then break, thereby releasing the stent against the vessel wall. However, because the sutures are wrapped around the circumference of the stent, once the suture fails, the entire circumference of the stent immediately and suddenly expands to contact the vessel wall.
Similarly, U.S. Pat. No. 7,022,132 to Kocur proposes biodegradable bands that are wrapped around the exterior of the stent or interwoven around the circumference of the stent. Kocur also proposes biodegradable bands wrapped around two individual stent struts. In each case, the bands are designed to initially hold the stent in a compressed shape. As the bands biodegrade they fracture, thereby immediately and suddenly releasing the stent against the vessel wall.
However, these techniques may unnecessarily traumatize the IEL by suddenly expanding against the vessel wall when the sutures or bands fail. Further, these techniques present significant manufacturing obstacles as the sutures and bands must be wound, woven, and/or tied around the circumference of the stent or individual stent struts to keep the stent or portions thereof in a compressed configuration. It has become apparent to the inventor that an improved stent would be desirable.